1. Field of the Invention
The present invention relates to a head position recognition method, a speed calculation method, and a head movement speed control device for controlling a data read/write head used to access a desired data track on a rotary recording medium.
2. Prior Art
The servo-side servo method whereby servo data written to one side of the recording medium is used to position the read/write head at the target data track is widely used with magnetic disk storage media. The drawback to this method, however, is that precise positioning of the data head to the required track is dependent upon the precise relationship between the servo and data side tracks. Because various environmental changes, including temperature changes inside the device, can easily disturb this relationship, resulting in offset tracks, it is difficult to increase the real track density.
The data-side servo method has therefore become more common because the servo data is written to the data side of the medium for improved read/write reliability. There are also different variations of the data-side servo method, one of which is the sector servo method.
In the sector servo method, a servo sector is reserved at the beginning of each data sector to store the positioning information. When the desired data track is selected, the servo data written to the servo sector is read to drive the data head over the data track. The drawback to this method, however, is that only tracking control data is written to the servo sector. It is therefore necessary to provide a separate position detection device to move the head at high speed, or to supply head positioning data from the servo side of the media. Furthermore, when sufficient positioning data is written to the servo sector to enable high speed disk access based on the servo sector data alone, the servo sectors occupy a higher percentage of total disk space than the data sectors, resulting in a significant drop in storage capacity.
An alternative method was therefore proposed for encoding the track number in the servo sector, and using this data for head control (Japanese Patent Laid-Open Number S51-131607). With this method, head speed is controlled by obtaining the position data (address) of the tracks passed by the head during the data seek operation at dispersed intervals, obtaining the average seek time between sectors and comparing this average with the command seek time. This method is now quite common because it can be realized with a simple mechanical design and yields good cost performance even when the number of data disks in the disk drive is few.
The drawback to this method (Japanese Patent Laid-Open Number S51-131607) of controlling the access speed by obtaining the average seek time between sectors from the track address is the control characteristics of the access speed. A method has also been proposed (Japanese Patent Application Number M2-39979) for improving these characteristics.
FIG. 9 is an example of the servo pattern of the servo sector proposed in Japanese Patent Laid-open H3-242882. The dispersed servo sectors embedded in the data tracks of a rotary storage medium are shown. The servo sector 2, data sector 3, and data head 5 are shown in FIG. 9. The servo sector 2 comprises a burst area 18 used to obtain the automatic gain control (AGC) signal, an erase area 19 for detecting the servo sector, a track code 20 for obtaining the track address, and a position address 21 for obtaining the offset of the data head 5 from the correct track position for tracking control. The erase area 19 is set to have the greatest possible erase time for the data tracks, and the position address 21 is composed of burst signals .alpha. and .beta. offset one-half track to the servo sector 2. The servo sectors are read-only.
The track code 20 is an embedded code comprising a synchronization bit S indicating the beginning of the track code 20, a zone discrimination component 20a comprising three dibit pattern A, B, and C identifying the guard zone, data zone, and type of data zone, a first servo pattern 20b comprising three-phase 3 tracks/cycle dibit pattern G, H, and I, a second servo pattern 20c comprising two dibit pattern D and E of 12 tracks/cycle with a three track offset, and a third servo pattern 20d comprising a dibit pattern F having 6 tracks/cycle and offset from the second servo pattern at least 1.5 tracks.
When reading or writing to the data track, the data head 5 follows a track that overlaps two adjacent servo sectors 2. The data head 5 is shown in FIG. 9 positioned at the sixth data track on the storage medium, resulting in the read signal waveform shown in FIG. 10. Specifically, through the servo sector the data head 5 obtains a "servo sector signal" corresponding to the servo patterns, specifically comprising a burst component 18, no-signal erase component 19, a track code 20 comprising the synchronization bit S, zone discrimination bits A, B, and C, second servo pattern positions D and E, third servo pattern position F, and first servo pattern positions G, H, and I, and a position address 21 component comprising the burst signals .alpha. and .beta.. Because the data head 5 follows a path overlapping half of two tracks through the servo sector, the signal output when the servo pattern is present on only one track is approximately half signal output when the servo pattern is present on both servo tracks.
Japanese Patent Laid-open H3-242882 describes a device and method for recognizing where the data head 5 is positioned in the 12 track/cycle servo pattern using the patterns D, E, F, G, H, and I in the playback waveforms A-I. The possible head position is first restricted to three of the twelve tracks by referencing a table or algorithm based on the digitized signals for D, E, and F (e.g., L, H, and H in the FIG. 10 waveform from the data head 5).
The peak values of the G, H, and I read signals are next stored, and the binary expression G&gt;H, H&gt;I, and I&gt;G (e.g., H, L, H for this data head 5) is generated. By again referencing a table or algorithm based on these digitized signals, the possible data head position is further restricted within the three candidate tracks to an area of 1/2 track width. The maximum and minimum peak hold signals are then selected from the G, H, and I signals based on the digitized data G&gt;H, H&gt;I, and I&gt;G (where the maximum is G and the minimum is H in this example).
An offset of approximately half the maximum peak hold value is then applied to the signal with the lowest peak value (H in this example). This signal increased by the offset value is then compared with the second highest peak value signal (in this example, H+ offset and I, respectively) using the equation (H+ offset)&gt;I. This comparison further restricts the data head position from the previously limited 1/2 track width to 1/4 track width. Using this result and the signals D, E, F, G, H, and I, it is possible to recognize the position of the data head in the 12 track/cycle servo pattern with 1/4 track resolution in the 48-part (=12 * 4) subtrack code.
The head positioning device shown in Japanese Patent Application H2-39979 positions the head in the target track using this head position recognition method. Specifically, when positioning the head to the target track with speed control, the position of the head in the 12 track/cycle servo pattern is recognized by sampling the playback signal of the servo pattern obtained each time the head passes the servo sector. Furthermore, each time the head passes the servo sector, the distance the head has travelled is obtained from the previously sampled head position and the currently detected head position, and the average seek time of the head between samples is further obtained by dividing this distance by the sample cycle.
When positioning the head to the target track with speed control, the target speed at which stable track access is possible is specified for each servo sector according to the position of the head relative to the target track. This speed control system is thus a feedback loop in which the difference between the target speed and the average seek speed of the head between samples is returned.
For example, it is assumed that the previously sampled head position is within the range of subtrack code 2 in the 12 track/cycle servo sector, and is currently positioned within the range of subtrack code 20. The average seek speed V of the head between samples can thus be expressed by equation 1 below EQU V=(STCn-STC.sub.0)*(N*Xtp/48)Ts (1)
where STCo is the subtrack code of the previous sample, STCn is the subtrack code of the current sample, Xtp is the track pitch, and Ts is the sampling cycle of the servo sector.
Thus, if Xtp=12 .mu.m and Ts=300 .mu.sec, the average seek speed of the head is V=18 cm/sec. The speed control loop is thus formed by obtaining the average seek speed of the head, reducing this to the target speed appropriate to the distance to the target track, and returning this value to the feedback loop.
As described above, however, the servo pattern of the invention described in Japanese Patent Laid-open H3-242882 has a 12 track cycle. Thus, when the distance travelled between sectors by the head is greater than twelve tracks, the average seek speed of the head between sectors cannot be correctly obtained.
For example, it is assumed that the previously sampled head position is within the range of subtrack code 2 in the 12 track/cycle servo sector during high speed control, and is currently positioned within the range of subtrack code 3. The average seek speed V of the head between samples must therefore be detected as V=49 cm/sec because the distance between the servo sectors is 48 subtrack codes +1=49 subtrack codes. If equation 1 is used to detect the average seek speed, however, the result will be V=1 cm/sec. Thus, because the target speed is calculated based on a falsely detected average seek speed, the speed control loop will fail and seek errors occur. Thus, if the distance of head travel between servo sectors is limited to 12 tracks, the maximum possible seek speed will be suppressed, and the seek time to the target track will increase.
For example, if the track pitch Xtp=12 .mu.m and the sample cycle Ts=300 .mu.sec, the maximum speed Vmax=48 cm/sec. With a standard 3.5" hard disk drive, however, the maximum seek speed is approximately 1 m/sec. because the required seek speed is typically the average seek speed plus several milliseconds. It is, of course, possible to increase the maximum seek speed by shortening the sampling cycle Ts, but this will increase the space required by the servo sector on the storage medium and reduce the data storage capacity of the drive. Furthermore, the maximum seek speed can also be increased by increasing the track pitch Xtp, but this will reduce the number of tracks on the storage medium and likewise reduce the data storage capacity of the drive. It follows that limiting the distance travelled by the head between servo sectors to twelve tracks limits the detection range of the head seek speed, and limits the improvements that can be achieved in the overall head positioning performance of the storage device.